1. Field of the Invention
The present invention relates to a surface mounting type planar magnetic device and production method thereof.
2. Description of the Related Art
In recent years, use of portable apparatuses driven by battery such as mobile phone and notebook-type personal computer has been accelerated. Since before, reduction of the weight and size of such portable apparatuses has been demanded and in addition to this demand, recently, higher functions such as communication function, display function and high-speed processing function for a large amount of information including image data have been also requested. Correspondingly, a demand for a power supply capable of transforming a single voltage from a battery to voltage levels necessary for various mounted devices such as CPU, LCD module, and communication power amplifier has been increased. Thus, to achieve the higher function as well as reduction of the size and weight, it has been an important theme to accelerate reductions of the size and thickness of such a magnetic device as transformer and inductor and the like to be mounted on the power supply.
Under this condition, a transformer or an inductor composed of sintered ferrite wound with coil is loaded on the conventional small portable apparatus. However, these components are difficult to thin thereby obstructing thinning of a power supply unit. A planar inductor composed of metal magnetic film layer, insulating layer, planar coil layer, insulating layer and metallic magnetic film layer on Si substrate in order to reduce the size and weight thereof has been described in Journal of the Magnetic Society of Japan 20(1996), pp. 922-924 and disclosed in Japanese Patent Application Laid-Open No. 4-363006. However, these conventional planar inductors have problems in terms of production cost and characteristic. That is, because metal magnetic film of 6 to 7 xcexcm is formed by spattering method and an insulating layer needs to be formed between the metal magnetic film and the planar coil, production cost for the planar inductor is sure to rise with respect to the conventional magnetic device.
The problems in terms of the characteristic are as follows. Because the planar inductor is driven by high frequency in MHz band, power loss is increased by generation of eddy current inside metal magnetic film which is electrically conductive. As for another characteristic problem, because upper and lower metal magnetic layers oppose each other through a slight nonmagnetic space, vertical alternate magnetic flux intersects the planar coil, so that eddy current is generated thereby increasing power loss. For the former, it has been proposed to divide the eddy current to small parts by forming a high resistance region on the same plane as the metal magnetic film according to Japanese Patent Application Laid-Open No. 6-77055 and for the latter, it has been proposed to divide the planar coil conductor to small parts according to Japanese Patent Application Laid-Open No. 9-134820 in order to improve the characteristics. However, it cannot be said that the characteristics have been improved sufficiently.
To solve these problems, Japanese Patent Application Laid-Open No. 11-26239 has disclosed a planar magnetic device employing a ferrite magnetic film formed by printing method or sheet method instead of the metal magnetic film. According to this method, magnetic paste produced by mixing binder with ferrite powder is printed on Si substrate and baked so as to produce a high resistance ferrite magnetic film. After a coil pattern is formed on this film by the plating method, ferrite magnetic film is formed thereon so as to produce a magnetic device. However, this publication has not disclosed an external electrode which is a feature of the present invention, and further surface mount technology (SMT) cannot be applied.
According to the conventional technology, because wire bonding method is employed to connect wires to a wired substrate, surface mount technology (SMT) cannot be applied, thereby leading to increase of production cost. The present invention intends to eliminate defects of the conventional technology and provide a surface mounting type magnetic device which achieves excellent characteristics at low cost. Concrete subjects of the present invention are as follows.
(a) Planar magnetic device capable of being thinned and mounted on the surface of a printed board
(b) Planar magnetic device having small power loss and large inductance
(c) Excellent frequency characteristic, small disparity of the characteristic and excellent reliability
The inventors had considered means for solving the above described problems sincerely and finally, completed the present invention by employing the following means. The concrete means will be described in details separately. These means are effective even if they are used independently and further a more conspicuous effect can be obtained by combining two or more means.
According to a first embodiment of the present invention, there is provided a surface mounting type planar magnetic device comprised of upper ferrite magnetic film, lower ferrite magnetic film and a planar coil interposed therebetween, in which an opening is formed in the upper ferrite magnetic film above the planar coil terminal portion and an external electrode conductive with the coil terminal portion through the opening is formed on the upper ferrite magnetic film.
The conventional planar magnetic device has a substrate for supporting the magnetic film and coil, which occupy most thickness of the device. According to the present invention, by composing the structure of the planar magnetic device with lower ferrite magnetic film, planar coil, upper ferrite magnetic film and external electrode while removing the substrate, the planar magnetic device can be thinned further. Further, because an external electrode is provided, the surface mount technology can be applied. An example of production of a magnetic device free of the substrate (substrate free magnetic device) will be described. This is just an example and the present invention is not restricted to this example. Lower ferrite magnetic film containing Cu is formed on a Si substrate and consequently, a planar coil, upper ferrite magnetic film and external electrode are formed. After that, if this is left under constant temperature and humidity of 90xc2x0 C., 95% RH (Relative Humidity) for more than 10 hours, for example, the ferrite magnetic film and substrate can be separated through an interface therebetween, so that the substrate free magnetic device can be obtained. If there occurs a trouble of handling upon actual use because it is too thin, this can be formed as a substrate provided magnetic device like conventionally and by adding an external electrode to this, the surface mounting type magnetic device may be produced.
According to a second embodiment of the present invention, there is provided a surface mounting type planar magnetic device wherein a lower ferrite magnetic film is formed on a substrate; a planar coil is formed on the lower ferrite magnetic film; an upper ferrite magnetic film having an opening above a terminal portion of the planar coil is formed; and an external electrode conductive with the planar coil terminal portion is formed. In this case, any substrate material can be used if it achieves a function as a supporting body. It is more preferable to use Si substrate or Al2O3 (alumina) substrate which are used in semiconductor industry in terms of cost performance.
In the surface mounting type planar magnetic device of the present invention, preferably, the planar coil is a spiral coil or a combination of plural spiral coils connected in series. Further, the planar coil is preferred to be composed of Cu conductor. The reasons will be described below.
Although for example, spiral type, meander type and the like can be employed for the planar coil, the spiral type is preferable because it is capable of achieving a larger inductance. Further, by arranging two or more spiral type coils in series such that they are connected, it is possible to obtain an inductance larger than times the inductance of a single coil by the number of the coils.
If the planar magnetic device is used as a chalk coil for the DC/DC converter, power loss Pi of the inductor is expressed in a following expression (1).                     Pi        =                              Rdc            ⁡                          (                                                I                  dc                  2                                +                                                      I                    p                    2                                    3                                            )                                +                      Rm            ⁢                                          I                p                2                            3                                                          (        1        )            
where
Rdc: DC resistance of coil
Rm: equivalent resistance of magnetic substance
Idc: DC current
Ip: peak current of triangular wave
Therefore, to reduce power loss of the inductor, it is preferable to decrease coil DC resistance Rdc, that is, use coil material having a small resistivity. Such material includes Ag (1.47xc3x9710xe2x88x926 xcexa9cm), Cu (1.55xc3x9710xe2x88x926 xcexa9cm), and Ni (6.2xc3x9710xe2x88x926 xcexa9cm). Although copper sulfate plating bath is employed for Cu, silver cyanide plating bath is employed for Ag, providing with a poor work efficiency. Further, Ag needs higher cost than Cu and has a problem in migration. The baking temperature of the upper ferrite magnetic film needs to be reduced because the melting point of Ag is 962xc2x0 C., which is lower than that of Cu of 1085xc2x0 C. Ni has a high risistivity. From standpoints of production and work efficiency, the Cu planar coil is preferred by the planar inductor employing the ferrite magnetic film.
Next, preferably, the planar coil is made of Cu conductor formed by electro plating with two-layer films, comprised of a film composed of a metal selected from a group of Nb, Ta, Mo, and W or an alloy composed of two or more of these metals formed by dry process such as spattering method and Cu film as plating foundation. The method for forming the Cu coil includes electro plating method, electroless deposition, printing/baking method and the like. Although the printing/baking method is often employed for chip inductor used for signal, this method has a problem that the resistivity is deteriorated because of mixture of binder component and incomplete baking. The electroless deposition method has a slower deposition speed than electro plating so that productivity is low and further, B or P is mixed from reducing agent, thereby increasing resistivity largely. On the other hand, the electro plating method has a high productivity and can obtain pure metal so that its resistivity is the smallest. Therefore, the Cu conductor coil formed by the electro plating method is more preferable for the magnetic device of the present invention. When the planar coil is formed by electro plating, a plating foundation is required as an electrode, because the ferrite magnetic film is electrically an insulator. As a result of accumulated considerations of plating foundation material from viewpoint of adhesion to the ferrite magnetic film and planar coil, it is found preferable to employ two-layer films composed of a film formed of a metal selected from elements such as Nb, Ta, Mo and W or an alloy composed of two or more of these metals and Cu film having an excellent adhesion with the planar coil. As for the layered film, the film formed of a metal selected from elements such as Nb, Ta, Mo and W or an alloy composed of two or more of these metals is disposed on the side of the lower ferrite magnetic film, while the Cu film is disposed on the side of the upper ferrite magnetic film.
Next, the sectional shape of the planar coil will be described. When the section of the planar coil is trapezoidal and it is assumed that the side contacting the lower ferrite magnetic film is a lower bottom and the side contacting the upper ferrite magnetic film is a upper bottom, according to the present invention, it is defined that lower bottomxe2x89xa7upper bottom is forward taper while lower bottom less than upper bottom is inverse taper. The forward taper includes a rectangular section composed of vertical sides. The section of the planar coil for the magnetic device of the present invention is preferred to be of forward taper. In case of the inverse taper, there is generated a problem in adhesion because a contact area with the lower ferrite layer is small and upon production, upper ferrite paste is not fed around the planar coil well, so that a gap is generated between the coil and ferrite magnetic film, thereby producing problem on production such as increased disparity of inductance. Because these problems can be solved by employing the forward taper for the section of the planar coil, the section of the planar coil is preferred to be of forward taper.
The thickness of the planar coil is 10 xcexcm or more to 100 xcexcm or less. To reduce loss by DC resistance of the coil, it is effective to enlarge a sectional area of the coil as well as reduce resistivity of coil material as described above. At this time, if the coil thickness is decreased, the coil width is increased. AC resistance R(f) of a N-turn coil under frequency f is expressed in the expression (2).                               R          ⁡                      (            f            )                          =                  Rdc          ⁡                      [                          1              +                                                                    4                    ⁢                                          xe2x80x83                                        ⁢                                          π                      2                                        ⁢                                          f                      2                                        ⁢                                          tcd                      4                                                                            12                    ⁢                                          xe2x80x83                                        ⁢                                          ρ                      2                                                                      ·                                                      ∑                                          xe2x80x83                                        ⁢                                          (                                                                        Bk                          2                                                ⁢                        lk                                            )                                                                                                  ∑                      lk                                        ⁢                                          xe2x80x83                                                                                            ]                                              (        2        )            
where:
Rdc: DC resistance of coil
tc: coil thickness
d: coil line width
xcfx81: resistivity of coil
Bk: effective value of vertical alternate magnetic flux of k-th coil line
lk: length of k-th coil line
Therefore, it is evident that increase of the coil line width d is not preferable because it induces increase of coil loss due to vertical alternate magnetic field. Further, increase of the coil line width d is not preferred because it increases inductor dimensions (occupied area of device). For the above reasons, the lower limit of planar coil thickness tc is set to 10 xcexcm. Stray capacitance C exists in the magnetic device due to coil(=electrode)/ferrite magnetic film (=dielectric material) of coil structure and resonates with inductance L thereby deteriorating frequency characteristic. The resonant frequency fr is expressed in the expression (3).
fr=1/(2xcfx80√{square root over (LC)})xe2x80x83xe2x80x83(3)
where
L: inductance
C: stray capacitance
To obtain an appropriate resonant frequency fr, the stray capacitance C has to be minimized. The stray capacitance C is proportional to electrode area and inversely proportional to a distance between electrodes. Because the stray capacitance C is increased if the coil thickness is increased, the coil interval may be increased correspondingly. However, there occurs a new problem such as magnetic field ripple in magnetic film. Considering all these matters, the upper limit of the thickness tc of the planar coil is determined to be 100 xcexcm.
Next, according to the present invention, preferably, insulating coating film composed of SiNx (1xe2x89xa6xxe2x89xa61.5), AlNy (0.8xe2x89xa6yxe2x89xa61.2), Al2O3 or multiple layers thereof having a thickness of 0.1 xcexctm or more to 10 xcexcm or less is formed on the surface of the planar coil excluding a top face of the terminal portion in order to suppress mixture of oxygen from outside. In the upper ferrite magnetic film baking process, oxidation of the planar coil is prevented in order to prevent loss of inductor due to increase of coil resistance. As a result of accumulated considerations, it is found preferable to form coating film composed of SiNx (1xe2x89xa6xxe2x89xa61.5), AlNx (0.8xe2x89xa6xxe2x89xa61.2), Al2O3 or multiple layers thereof having a thickness of 0.1 xcexcm or more to 10 xcexcm or less so as to prevent mixture of oxygen into the surface of the planar coil. At this time, if the thickness of the coating film is 0.1 xcexcm or more, diffusion of oxygen to the Cu coil can be prevented. However, if the thickness exceeds 10 xcexcm, separation of the coating film occurs and a non-magnetic gap is generated between the coating film and the upper ferrite magnetic film. As a result, reduction of inductance, increase of loss accompanied by an increase of vertical alternate magnetic field intersecting the coil and the like occur. Therefore, the thickness of the coating film is preferred to be 0.1 xcexcm to 10 xcexcm.
Next, the composition of the ferrite magnetic film will be described. Preferably, the average composition of the ferrite magnetic film is Fe2O3: 40 to 50 mol %, ZnO: 15 to 35 mol %, CuO: 0 to 20 mol %, Bi2O3: 0 to 10 mol % while remainder is NiO or unavoidable impurity. This composition is average values for the entire magnetic device and it is permissible to select an optimum composition for the upper ferrite magnetic film, lower ferrite magnetic film, ferrite/substrate interface and the like, depending on a target position. The reason why the composition of the magnetic film is limited is as follows.
Fe2O3: 40 to 50 mol %
If Fe2O3 exceeds 50 mol %, electric resistance drops rapidly due to existence of Fe2+ ion. Reduction of electric resistance increases loss in the ferrite core rapidly due to eddy current which is generated when used in high frequency region. Fe2O3 is set to 40 to 50 mol % because deterioration of inductance is increased accompanied by drop of permeability of the ferrite magnetic film when Fe2O3 is less than 40 mol %.
ZnO: 15 to 35 mol %
ZnO affects inductance and Curie temperature largely. The Curie temperature is an important parameter which determines heat resisting property of the magnetic device. Although the Curie temperature is high when ZnO is less than 15 mol %, inductance drops. On the other hand, if ZnO exceeds 35 mol %, the Curie temperature Tc drops although inductance is high. Therefore, ZnO is preferred to be limited to 15 to 30 mol %.
CuO: 0 to 20 mol %
CuO is added to lower the baking temperature. Although the baking temperature drops if CuO exceeds 20 mol %, inductance deteriorates. Thus, the upper limit of CuO is set to 20 mol %.
Bi2O3: 0 to 10 mol %
Bi2O3 has an effect of reducing the baking temperature like CuO. If it exceeds 10 mol %, inductance deteriorates although the baking temperature drops. Therefore, the upper limit is set to 10 mol %.
Next, the thickness of the aforementioned lower ferrite magnetic film will be described. Inductance of the magnetic device depends on xcexcrxc3x97tm and xcexcrxc3x97tmxe2x89xa71000 (xcexcm) is required. where xcexcr is relative permeability and tm is film thickness. Considering that the permeability of the ferrite magnetic film in the magnetic device is 100-200, the film thickness needs to be 10 xcexcm or more. On the other hand, if the thickness of the lower ferrite magnetic film exceeds 100 xcexcm, inductance is increased. However, the film thickness is increased so that defects such as separation of the ferrite magnetic film often occur. Therefore, preferably, the thickness of the lower ferrite magnetic film is 10 xcexcm or more to 100 xcexcm or less.
Next, in the surface mounting type planar magnetic device having a substrate of the present invention, the concentration of CuO in a layer contacting the substrate in the lower ferrite magnetic film is 5 mol % or less and the concentration of CuO in other portions is more than 5 mol %. In case where the substrate of the lower ferrite magnetic film is Si, if a large amount of Cu is contained in the ferrite magnetic film, adhesion performance may drop. As a result of accumulated considerations on a way for solving this problem, it was found that a phase rich in Sixe2x80x94Cu deposited on ferrite/substrate interface reduced the adhesion performance and by suppressing this deposition amount, the reduction of the adhesion performance could be solved. That is, by reducing the concentration of CuO in the ferrite layer near the interface in contact with the Si substrate to less than 5 mol %, deposition on the phase rich in Sixe2x80x94Cu can be suppressed largely, thereby improving adhesion with the Si substrate. Baking at a reduced baking temperature with the concentration of CuO in the lower ferrite composition excluding near the ferrite/substrate interface is more preferable from viewpoints of prevention of warp of the substrate. An example of the method for achieving the lower ferrite film having such a structure will be described. A lower ferrite magnetic film of 5 mol % or less in concentration of CuO is formed on the Si substrate and the film thickness is several xcexcm after baking. Subsequently, ferrite magnetic film of more than 5 mol % in concentration of CuO is formed in a required thickness. At this time, although the two-layer ferrite magnetic films may be baked at the same time or separately (optimum baking temperature for each layer) twice, higher adhesion performance can be obtained if the baking is carried out separately. The above described matter is just an example and the present invention is not restricted to this.
Next, the lower ferrite magnetic film formed on the substrate is baked together with the substrate at 900xc2x0 C. to 1250xc2x0 C., it is cooled down to room temperature. If Si is employed for the substrate, a warp occurs in the substrate/lower ferrite magnetic film composite material because the thermal expansion of the ferrite magnetic film is 9-10xc3x9710xe2x88x926/K although the thermal expansion of the substrate is 2.4xc3x9710xe2x88x926/K. As a result, a trouble may occur in post process such as planar coil production step. This problem can be solved by introducing cracks into the ferrite magnetic film positively so as to reduce an area surrounded by the cracks. In the ferrite magnetic film for the magnetic device formed on the substrate, preferably, a number of cracks are formed at least on the surface of ferrite magnetic film on an opposite side not in contact with the substrate and an average of diameters of circles converted from the areas surrounded by the cracks is less than 100 xcexcm. Meanwhile, if a crack is produced in the ferrite magnetic film, the crack reaches an edge of the film. If it is intended to just restore the warp, this can be also achieved by introducing several cracks. However, if the crack interval is large so that the area surrounded by the cracks is increased, leaking magnetic flux is generated, thereby producing new problems such as reduction of inductance by diamagnetic field and separation of the ferrite magnetic film. For the reason, the number of cracks is increased so as to reduce each area surrounded by the cracks. Consequently, distortion generated in the ferrite magnetic film is released so that the aforementioned problem never occurs. The area of a portion surrounded by the cracks is expressed by equivalent diameter. The equivalent diameter refers to the diameter of a circle converted from the area of a portion surrounded by the cracks. If the average of the equivalent diameter of each portion surrounded by the cracks exceeds 100 xcexcm, the aforementioned leaking magnetic flux occurs or the ferrite magnetic film becomes likely to be separated. Therefore, the upper limit is set to 100 xcexcm or less. Meanwhile, the depth of the crack may be only in the surface of the film or may reach the surface of the substrate. Although a method for generating such a crack is not restricted to any particular one, but the crack may be generated by reducing the baking temperature to a temperature lower than usually, for example, not more than 920xc2x0 C. or increasing the cooling velocity so as to be higher than 5xc2x0 C. per minute. Further, it is permissible to add additional material for reducing grain boundary strength, for example, V2O5, In2O3 into film so as to reduce mechanical strength of the film.
An opening is made in the planar coil terminal portion of the upper ferrite magnetic film so as to be conductive with an external electrode, in order to prevent a conductor portion following the planar coil terminal portion from being exposed and being short-circuited with other coil portion except the coil terminal. The opening is preferred to be made inside by 50 xcexcm or more to 200 xcexcm or less from the periphery of the coil terminal portion and more preferred to be made inside by 100 xcexcm or more to 200 xcexcm or less. If a contact area with the external electrode is reduced too much, local heat is generated at the contact portion, thereby leading to such troubles as reduction of power efficiency at power supply and melt-down of the coil at worst. Therefore, area of the contact portion of the opening with the coil terminal portion is preferred to be 100 xcexcm2 or more.
The external electrode is disposed in the opening in the upper ferrite magnetic film. Preferably, this external electrode is formed by treating conductor paste composed of mainly one of Ni, Pd, Pt, Ag, Au or alloy powder containing these materials or solder paste composed of mainly Sn by heat treatment. Although an example of production method will be described about the conductor paste and solder paste, the present invention is not restricted to this example. In case of the conductor paste, after printing, it is baked at 700 to 950xc2x0 C. At this time, it may be baked together with the upper ferrite magnetic film at the same time. On the other hand, for example, the solder paste has composition of 37Pb+63Sn, 90Pb+10Sn, 95Pb+5Sn. This solder paste is printed on the opening and melted by passing through a solder reflow furnace at 180 to 350xc2x0 C. so as to produce the external electrode.
Meanwhile, a metal cap may be mounted on the external electrode formed on the upper ferrite magnetic film so that it is joined to the planar coil terminal by heat treatment.
If the external electrode is formed so as to be in contact with at least one side or two sides if possible of the device end portion, circuit wiring in the surface mounting type planar magnetic device is simplified preferably. As a means for connecting the planar magnetic device of the present invention to the circuit substrate, an example of soldering in the solder reflow process will be described in embodiments below. It is permissible to employ other connecting method such as wire bonding method and bump connection method to connect the external electrode of the planar magnetic device to a connecting terminal of the circuit substrate.
A completed product is produced by attaching the external electrode to the terminal portion of the planar coil. If the surface of the coil terminal portion is contaminated in halfway process, conduction failure is likely to occur. At this time, local heat is generated at a defective portion, thereby power efficiency at the power supply dropping or at worst a fatal trouble being generated such as destruction of the magnetic device. To prevent such troubles, preferably, a process for treating the surface of the coil terminal portion, that is, a process for light etching with acid or a process for washing with organic solvent is entered before a process for providing with the external electrode. Although as cleaning agent, for example, mixed acid such as Acetone, phosphoric acid, acetic acid, nitric acid and organic solvent such as dimethyl sulfoxide and N-methyl-2-pyrolidone may be used, the cleaning agent is not restricted to these. Because the baking of the upper ferrite magnetic film is carried out with the Cu coil existing inside, it is an important matter to prevent oxidation of the Cu during the baking. Although formation of coating film on the surface of the Cu coil is an effective means, the oxidation of the coil can be prevented by baking at 900xc2x0 C. or more to 1050xc2x0 C. or less in the atmosphere in which the concentration of oxygen is less than 1 vol. % after the upper ferrite magnetic film is applied. If the concentration of oxygen is less than 1 vol. %, the ferrite magnetic film can be baked without deteriorating DC resistance of the copper coil largely. At this time, if the temperature exceeds 1050xc2x0 C., it is near the melting point of the copper coil, thereby inducing a change of the coil configuration or at worst melting-down of the coil. On the other hand, if the baking temperature is less than 900xc2x0 C., the baking of the ferrite magnetic film is not accelerated sufficiently, so that a large inductance is not obtained and film strength is weakened. For the reasons, preferably, the concentration of oxygen in the atmosphere is less than 1 vol. % and the baking temperature is 900xc2x0 C. or more to 1050xc2x0 C. or less.